Lightly grease an 8x8 dish with vegetable shortening or butter and line the pan with parchment paper. Cut the paper long enough to overhang the two edges of the pan.
In a large microwaveable bowl or 8 cup glass measure, combine the butter, brown sugar and corn syrup. Microwave at 100% power for 2 minutes.
Stir mixture well. It will be a little grainy. Microwave for another 2 minutes, stir well.
Add 2/3 cup sweetened condensed milk. Microwave for 3 minutes. Watch carefully, it is prone to bubble up at this stage. If it starts to bubble over, open the microwave door intermittently so that the mixture will sink down and not bubble over.
Remove from microwave and stir in vanilla extract. Be very careful at this stage, the caramel is extremely hot and sticky!
Pour hot caramel into the parchment lined pan.
Lightly sprinkle with grey sea salt.
Let stand until set, or speed setting with refrigeration. Lift caramel block from pan and cut into individual squares.
Wrap the caramel pieces in waxed paper squares.
This recipe should be made in a standard household microwave. High powered commercial microwaves are not suitable for this recipe.